The candela (/kænˈdɛlə/ or /kænˈdiːlə/; symbol: cd) is the
base unit of luminous intensity in the International System of Units
(SI); that is, luminous power per unit solid angle emitted by a point
light source in a particular direction.
Luminous intensityLuminous intensity is
analogous to radiant intensity, but instead of simply adding up the
contributions of every wavelength of light in the source's spectrum,
the contribution of each wavelength is weighted by the standard
luminosity function (a model of the sensitivity of the human eye to
different wavelengths).[4][5] A common wax candle emits light with a
luminous intensity of roughly one candela. If emission in some
directions is blocked by an opaque barrier, the emission would still
be approximately one candela in the directions that are not obscured.
The word candela means candle in Latin.

Definition[edit]
Like most other SI base units, the candela has an operational
definition—it is defined by a description of a physical process that
will produce one candela of luminous intensity. Since the 16th General
Conference on Weights and Measures (CGPM) in 1979, the candela
has been defined as:[6]

The candela is the luminous intensity, in a given direction, of a
source that emits monochromatic radiation of frequency
7014540000000000000♠540×1012 hertz and that has a radiant
intensity in that direction of 1/683 watt per steradian.

The definition describes how to produce a light source that (by
definition) emits one candela, but does not specify the luminosity
function for weighting radiation at other frequencies. Such a source
could then be used to calibrate instruments designed to measure
luminous intensity with reference to a specified luminosity function.
An appendix to the SI Brochure[7] makes it clear that the luminosity
function is not uniquely specified, but must be selected to fully
define the candela.
The candela is sometimes still called by the old name candle,[8] such
as in foot-candle and the modern definition of candlepower.
Explanation[edit]
The frequency chosen is in the visible spectrum near green,
corresponding to a wavelength of about 555 nanometres. The human
eye, when adapted for bright conditions, is most sensitive near this
frequency. At other frequencies, more radiant intensity is required to
achieve the same luminous intensity, according to the frequency
response of the human eye. The luminous intensity for light of a
particular wavelength λ is given by

where Iv(λ) is the luminous intensity, Ie(λ) is the radiant
intensity and

y
¯

(
λ
)

displaystyle textstyle overline y (lambda )

is the photopic luminosity function. If more than one wavelength is
present (as is usually the case), one must integrate over the spectrum
of wavelengths to get the total luminous intensity.
Examples[edit]

A common candle emits light with roughly 1 cd luminous intensity.
A 25 W compact fluorescent light bulb puts out around
1700 lumens; if that light is radiated equally in all directions
(i.e. over 4π steradians), it will have an intensity of

.
Focused into a 20° beam, the same light bulb would have an
intensity of around 18,000 cd within the beam.
The luminous intensity of light-emitting diodes is measured in
millicandelas (mcd), or thousandths of a candela. Indicator LEDs are
typically in the 50 mcd range; "ultra-bright" LEDs can reach
15,000 mcd (15 cd), or higher.

Origin[edit]
Prior to 1948, various standards for luminous intensity were in use in
a number of countries. These were typically based on the brightness of
the flame from a "standard candle" of defined composition, or the
brightness of an incandescent filament of specific design. One of the
best-known of these was the English standard of candlepower. One
candlepower was the light produced by a pure spermaceti candle
weighing one sixth of a pound and burning at a rate of 120 grains
per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a
unit based on the output of a Hefner lamp.[9]
It became clear that a better-defined unit was needed. Jules Violle
had proposed a standard based on the light emitted by 1 cm2 of
platinum at its melting point (or freezing point), calling this the
Violle. The light intensity was due to the
Planck radiatorPlanck radiator (a black
body) effect, and was thus independent of the construction of the
device. This made it easy for anyone to measure the standard, as
high-purity platinum was widely available and easily prepared.
The
Commission Internationale de l'ÉclairageCommission Internationale de l'Éclairage (International
Commission on Illumination) and the CIPM proposed a “new candle”
based on this basic concept. However, the value of the new unit was
chosen to make it similar to the earlier unit candlepower by dividing
the Violle by 60. The decision was promulgated by the CIPM in 1946:

The value of the new candle is such that the brightness of the full
radiator at the temperature of solidification of platinum is
60 new candles per square centimetre.[10]

It was then ratified in 1948 by the 9th CGPM which adopted a new
name for this unit, the candela. In 1967 the 13th CGPM removed the
term "new candle" and gave an amended version of the candela
definition, specifying the atmospheric pressure applied to the
freezing platinum:

The candela is the luminous intensity, in the perpendicular direction,
of a surface of 1 / 600 000 square metre of a black body at
the temperature of freezing platinum under a pressure of
101 325 newtons per square metre.[11]

In 1979, because of the difficulties in realizing a
Planck radiatorPlanck radiator at
high temperatures and the new possibilities offered by radiometry, the
16th CGPM adopted the modern definition of the candela.[12] The
arbitrary (1/683) term was chosen so that the new definition would
precisely match the old definition. Although the candela is now
defined in terms of the second (an SI base unit) and the watt (a
derived SI unit), the candela remains a base unit of the SI system, by
definition.[13]
SI photometric light units[edit]

SI photometry quantities

v
t
e

Quantity
Unit
Dimension
Notes

Name
Symbol[nb 1]
Name
Symbol
Symbol[nb 2]

Luminous energy
Qv [nb 3]
lumen second
lm⋅s
T⋅J
The lumen second is sometimes called the talbot.

^ Standards organizations recommend that photometric quantities be
denoted with a suffix "v" (for "visual") to avoid confusion with
radiometric or photon quantities. For example: USA Standard Letter
Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^ The symbols in this column denote dimensions; "L", "T" and "J" are
for length, time and luminous intensity respectively, not the symbols
for the units litre, tesla and joule.
^ a b c Alternative symbols sometimes seen: W for luminous energy, P
or F for luminous flux, and ρ or K for luminous efficacy.

Relationships between luminous intensity, luminous flux, and
illuminance[edit]
If a source emits a known luminous intensity Iv (in candelas) in a
well-defined cone, the total luminous flux Φv in lumens is given by

Φv = Iv 2π [1 − cos(A/2)],

where A is the radiation angle of the lamp—the full vertex angle of
the emission cone. For example, a lamp that emits 590 cd with a
radiation angle of 40° emits about 224 lumens. See
MR16MR16 for
emission angles of some common lamps.[14][15]
If the source emits light uniformly in all directions, the flux can be
found by multiplying the intensity by 4π: a uniform 1 candela
source emits 12.6 lumens.
For the purpose of measuring illumination, the candela is not a
practical unit, as it only applies to idealized point light sources,
each approximated by a source small compared to the distance from
which its luminous radiation is measured, also assuming that it is
done so in the absence of other light sources. What gets directly
measured by a light meter is incident light on a sensor of finite
area, i.e. illuminance in lm/m2 (lux). However, if designing
illumination from many point light sources, like light bulbs, of known
approximate omnidirectionally-uniform intensities, the contributions
to illuminance from incoherent light being additive, it is
mathematically estimated as follows. If ri is the position of the i-th
source of uniform intensity Ii, and â is the unit vector normal to
the illuminated elemental opaque area dA being measured, and provided
that all light sources lie in the same half-space divided by the plane
of this area,